1. Field of the Invention
This invention relates to an optical recording medium which is capable of accomplishing recording and reproduction by a light beam.
2. Description of the Prior Art
Thin films of alloys of rare earth-transition metals, thin films of reductive oxides such as chalcogen compounds utilizing the phase shift from non-crystalloid to crystalloid, heat mode recording mediums, thermoplastic recording mediums, etc. are known as optical recording medium used in optical discs. For example, as magneto-optical recording mediums formed by thin films of alloys of rare earth-transition metals, there are known polycrystal thin films such as MnBi and MnCuBi, non-crystalloid or amorphous thin films such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo and GdTbCo, and single crystal thin films such as GIG (gadolinium iron garnet).
Of these thin films, the non-crystalloid thin films have recently been considered to be excellent as magneto-optical recording mediums with the film formability with which a thin film of a large area is made at a temperature in the vicinity of room temperature, the writing efficiency for writing a signal with small light-heat energy and the reading efficiency for reading out the written signal at a good S/N ratio being taken into account. Particularly, GdTbFe having a curie temperature of the order of 150.degree.-200.degree. C. and GdTbFeCo having a great Kerr rotation angle and excellent in reproducing performance (Japanese Laid-open Patent Application No. 196639/1983 corresponding to U.S. application Ser. No. 820,828) are suitable for a magneto-optical recording medium.
However, non-crystalloid magnetic materials, including GdTbFe, are generally inferior in corrosion resistance and suffer from a disadvantage that they are corroded in an atmosphere having moisture and become deteriorated in magnetic characteristic. Particularly, in a construction in which a reflecting layer or an interference layer and a reflecting layer are provided on the back side of the magnetic recording layer (that side opposite to the side on which light is incident) for the purpose of improving the S/N ratio, the thickness of the magnetic recording layer must be made small from the necessity of effectively utilizing the Faraday effect and therefore, the corrosion resistance is more aggravated. Such deterioration of the recording characteristic resulting from oxidation has been a disadvantage common to magneto-optical recording mediums and optical recording mediums.
In order to eliminate such a disadvantage, there have been proposed disc-like optical recording mediums of a structure in which various protective layers are provided on an optical recording layer, or of the air sandwich structure in which a magnetic recording layer is enveloped by inert gas, or of the cemented structure in which another substrate is cemented to a protective layer with an adhesive layer interposed therebetween. In any of these, however, the protecting effect has been insufficient in a case where the recording layer is formed of a readily oxidizable material or where the thickness of the recording layer is very small.